Research to evaluate conditions for the safe and effective use of current pesticide chemicals and to develop better pesticides with improved selectivity continues to focus on their metabolism, photodecomposition and mode of action. The major emphasis is on pyrethroid insecticides, which are possible replacements in agricultural pest control for the chlorinated hydrocarbons that have unfavorable persistence and toxicological features. Chemical studies involve radiosynthesis, photodecomposition, synthesis of metabolites and photoproducts, and spectroscopy and analysis of the pyrethroids and their degradation products. Toxicological investigations on pyrethroids concern their metabolism in mammals, insects, plants and enzyme systems and synergistic interactions with other pesticides. Studies on organophosphorus and methylcarbamate pesticides emphasize the biochemical lesions leading to teratogenic and/or delayed neurotoxic effects in vertebrates and their mode of action as herbicides and fungicides. Thiocarbamate herbicide metabolism is examined in relation to residues, selective toxicity, antidote action, and the mechanism by which they inhibit weed growth. Noncholinergic effects of highly toxic bicyclic phosphorus compounds are also studied. Trisubstituted-organotin biocides are subjected to radiosynthesis, photochemical investigations, and synthesis of carbon-hydroxylated derivatives for use in metabolite identifications. The metabolites formed in microsomal mixed-function oxidase systems and in mammals are compared with the respective parent compounds as to their toxicology and mode of action. Other pesticidal inhibitors and uncouplers of mitochondrial electron transport and inhibitors of microsomal oxidases are examined as to their metabolic and degradation chemistry. Studies on other toxicants emphasize the identification, analysis, metabolism and toxicology of toxaphene components and the mode of action of insect growth regulators including those that inhibit chitin synthesis. Attention is also given to the metabolism and mode of action of pesticidal natural products and new pesticides from industrial screening programs.